I have a 2 floor house that the 2nd flr is sagging. I have opened the ceiling under the sag and there's 3 2x8 nailed together as a header.my question the 2x8 header is appx 13 feet long is this to long for this size header or do I have another problem?

By looking at the IRC code book table R502.5(2), headers and girders supporting one floor only: 3-2x8 span should be depending on the building width, from 5'-7" to 7'-2".

For supporting 2 floors: 3-2x8 span should be anywhere from 3'-11" to 5'-1" depending on the building width.

These are for interior headers.
your header is undersized and to long for the application it was intended for.